A method for a radioimmunoassay of human renin substrate (angiotensinogen) is proposed. The research proposal involves the selective cleavage of a peptide moiety from the human renin substrate molecule, and the conversion of this peptide into a suitable antigenic substance by chemical modification. The antibody formed to this antigen will be utilized in the conventional methods of radioimmunoassay. By use of this method, plasma renin substrate concentrations in various clinical disorders will be determined and evaluated.